Suzanne de Bruijn

14 Chapter 1.1 eye. Light perception is a multi-step process which involves transmission and refraction of light rays, allowing these signals to reach the retina, the conversion of light energy into electrical signals and the refinement of these signals within the neural circuits of the retina and the brain. 5 The eye and especially the retina are complex multi-layered structures, all designed to allow optimal image acquisition of our surroundings. Anatomy of the eye On the exterior side of the eyeball several anatomical structures can be distinguished: the pupil, the iris and the cornea. 6,7 The pupil is a black-looking aperture that allows light rays to enter the eye. It is surrounded by the iris, a colored circular muscle that determines the color of the eyes. This circular muscle controls the size of the pupil, and the amount of light that enters the eye. The cornea, on the other hand, is a transparent external surface that covers both the pupil and the iris and functions as a powerful lens of the optical system. 7 On the interior side, the eye is a fluid-filled sphere enclosed by three layers of tissues: an external, intermediate and internal layer ( Figure 1A ). The outermost layer consists of the sclera and the cornea. The sclera is a continuation of the cornea, and is a tough white fibrous tissue which forms a strong enclosure of the eye and provides protection for the inner layers. The intermediate tissue layer consists of a posterior (choroid) and an anterior part (iris and ciliary body). The choroid is the vascular wall of the eye, and provides nutrition for the sensory cells located in the inner tissue layer. The ciliary body is a ring of tissue that encircles the lens and has a muscular and vascular component that are required for adjustment of the refractive power of the lens and ciliary processes, respectively. The lens is a transparent body that is located behind the iris. Together with the cornea, the lens is responsible for refraction of the light that allows the creation of a sharp, focused image on the back of the eye. The muscles of the ciliary body act to flatten or thicken the lens, allowing focused visualization of objects observed at variable distances in a process called accommodation. The innermost tissue layer is the retina and is the sensory part of the eye. It contains the light-sensitive cells, called photoreceptors, and houses the biochemical processes that are initiated by light perception. This will ultimately lead to the transmission of signals via the optic nerve through the optic disc, directed to the visual cortex of the brain. 5-7 Besides the different tissues, there are three different fluid compartments within the eyeball ( Figure 1A ). An anterior chamber, that is located between the cornea and the iris, a posterior chamber between the iris and the lens, and a vitreous chamber between the lens and the retina. The anterior and posterior chambers are filled with aqueous humor, whereas the vitreous chamber is filled with vitreous humor. Aqueous humor